The natural inhibitors of coagulation, antithrombin and TFPI, are pseudosubstrates with high affinity for their specific target enzymes. Antithrombin is a single-chain globular molecule which depends on heparin to obtain its optimal inhibitory conformation required for docking and locking the catalytic centre ofits target enzymes, thrombin andfactorXa. Itforms 1:1 stoichiometric complexes which are rapidly cleared from the circulation. TFPI is a single-chain molecule with three Kunitz domains, which contain about 58 residues and three characteristic disulphide bonds. They act as pseudosubstrates for their target serine proteases. The first Kunitz domain of TFPI inhibits the factor Vila/tissue factor complex, whereas the second inhibits factor Xa; the function of the third Kunitz domain is unknown.

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